Python is a open source software. It has thousands of third-party packages developed by developers all over the world. All these packages were maintained by PiPy.

For developing complex application we need to install many third-party libraries based on the requirement. All these packages were installed in system level.
If we are working on two projects in parallel one project is dependent on python2.7 and other is dependent on python3.5.

After completion of development the installed packages are no more required.
And they may slowdown the working speed of the system. To avoid such conditions we use python virtual environment.